Phenolic resins include 1) thermoplastic novolac-type phenolic resins and 2) thermosetting resole resins. Novolac-type phenolic resins are used as a thermosetting phenolic resin composition by containing a curing agent (in most cases hexamethylenetetramine is used as a curing agent). They have various excellent properties such as heat resistance, mechanical strength, electrical properties and so on, and are used for various applications such as molding compounds, laminated sheets, adhesives and so on.
In contrast, due to the low temperature dependence of the curing rate, conventional thermosetting phenolic resin compositions have a problem as follows. In a molding method such as injection molding for example, if a resin composition having a fast curing rate at the molding temperature is used for molding in order to reduce production time in a molding process, curing of the melted resin is promoted at a temperature lower than the molding temperature when melting the resin composition. Consequently, thermal stability of the resin deteriorates and it causes a problem that the range of conditions required for molding is strictly limited so as to impair molding workability. On the other hand, in view of thermal stability of resin, if a resin composition having a low curing rate at a temperature lower than the molding temperature at which the resin composition is melted is used, the curing rate at the molding temperature also decreases so as to reduce productivity. Thus, a thermosetting resin composition which is well balanced between productivity and molding workability, has a fast curing rate at the molding temperature, enables to reduce production time, and is stable without curing at a temperature lower than the molding temperature at which the resin composition is melted has been strongly desired.
In such a resin composition, in order to increase the curing rate of such a thermosetting phenolic resin composition, a high molecular novolac-type phenolic resin (e.g. pages 2 to 4 of Japanese Patent Application laid-open (JP-A) No. 2001-40177) or a high ortho novolac-type phenolic resin (e.g. pages 2 to 3 of JP-A No. Hei. 8-302158) has been used hitherto; however, effects thereof are insufficient.
Adding an organic acid as a decomposition accelerator of hexamethylenetetramine, which is used as a curing agent, has been proposed as a method to increase the curing rate; however, effects thereof are also insufficient.